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Gravitation
About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.
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TL;DR: In this article, the effects of low quantum gravity scale on string excited states of standard model particles were studied in a simple string model and the effect of these states on real and virtual graviton emission was analyzed.
Abstract: Arkani-Hamed, Dimopoulos, and Dvali have proposed that the fundamental gravitational scale is close to 1 TeV, and that the observed weakness of gravity at long distances is explained by the presence of large extra compact dimensions. If this scenario is realized in a string theory of quantum gravity, the string excited states of standard model particles will also have TeV masses. These states will be visible to experiment and in fact provide the first signatures of the presence of a low quantum gravity scale. Their presence also affects the more familiar signatures due to real and virtual graviton emission. We study the effects of these states in a simple string model. (c) 2000 The American Physical Society.
220 citations
TL;DR: In this article, it was shown that the long-wavelength structure of quantum gravitation is the same as that of the classical Einstein action, without assuming the existence of an underlying tensor field.
219 citations
TL;DR: In this paper, it is shown that several gravitating structures like stars, spiral galaxies, elliptical galaxies and clusters of galaxies can be self-consistently described without asking for dark matter.
Abstract: An alternative view to the dark matter puzzle is represented by Extended Theories of Gravity. The approach consists in addressing issues like dark components from the point of view of gravitational field instead of requiring new material ingredients that, up to now, have not been detected at fundamental level. In this review paper, by extending the Hilbert-Einstein action of gravitational field to more general actions (e.g. f(R) gravity), it is shown that several gravitating structures like stars, spiral galaxies, elliptical galaxies and clusters of galaxies can be self-consistently described without asking for dark matter. It is also shown that standard General Relativity tests and Equivalence Principle constraints can be evaded at Solar System scales.
219 citations
TL;DR: The evidence that general relativistic effects may cause the stars to individually collapse into black holes prior to merging and the strong fields cause the last stable orbit to occur at a larger separation distance and lower frequency than previously estimated is discussed.
Abstract: We describe a numerical method for calculating the (3+1)-dimensional general relativistic hydrodynamics of a coalescing neutron-star binary system. The relativistic field equations are solved at each time slice with a spatial three-metric chosen to be conformally flat. Against this solution to the general relativistic field equations, the hydrodynamic variables and gravitational radiation are allowed to respond. The gravitational radiation signal is derived via a multipole expansion of the metric perturbation to the hexadecapole ({ital l}=4) order including both mass and current moments and a correction for the slow-motion approximation. Using this expansion, the effect of gravitational radiation on the system evolution can also be recovered by introducing an acceleration term in the matter evolution. In the present work we illustrate the method by applying this model to evaluate various orbits of two neutron stars with a gravitational mass of 1.45{ital M}{sub {circle_dot}} near the time of the final merger. We discuss the evidence that, for a realistic neutron-star equation of state, general relativistic effects may cause the stars to individually collapse into black holes prior to merging. Also, the strong fields cause the last stable orbit to occur at a larger separation distance and lower frequency than previously estimated. {copyright}more » {ital 1996 The American Physical Society.}« less
219 citations
TL;DR: It is shown how the operators that naturally lie at the cutoff scale can affect the speed of propagation of gravitational waves and bring it back to unity at LIGO scales in a simple model with a known partial UV completion.
Abstract: The recent direct detection of gravitational waves from a neutron star merger with optical counterpart has been used to severely constrain models of dark energy that typically predict a modification of the gravitational wave speed. However, the energy scales observed at LIGO, and the particular frequency of the neutron star event, lie very close to the strong coupling scale or cutoff associated with many dark energy models. While it is true that at very low energies one expects gravitational waves to travel at a speed different than light in these models, the same is no longer necessarily true as one reaches energy scales close to the cutoff. We show explicitly how this occurs in a simple model with a known partial UV completion. Within the context of Horndeski, we show how the operators that naturally lie at the cutoff scale can affect the speed of propagation of gravitational waves and bring it back to unity at LIGO scales. We discuss how further missions including LISA and PTAs could play an essential role in testing such models.
219 citations